In mechanical timepieces, the material forming the balance spring is generally made from a metal alloy such as steel capable of remanent magnetisation if subjected to an external magnetic field. Although it is possible to envisage making the part in a non-magnetic material in order to neutralise the impact of an external magnetic field interfering with the operation of a mechanical timepiece of this type, the excellent mechanical qualities (ductility, elasticity, thermal expansion coefficient, etc.) of a balance spring made of conventional materials largely compensates for this drawback of magnetic sensitivity. Consequently, it has been sought to protect this type of balance spring from interfering fields, such that if the movement is subjected to a field of around 4.8 kA/m, the variation in rate does not exceed 30 seconds per day to comply with horological standards. Beyond this 4.8 kA/m field and without protection, the variation in rate is very variable and reaches significant variations of up to several minutes per day. This variation is due above all to the longitudinal magnetisation of the coils forming the balance spring, which produces torque on the balance axis to which the balance spring is connected. The torque is added to or subtracted from the normal mechanical torque. The variation in rate is also influenced, but to a lesser extent, by magnetostriction which tends to lengthen or shorten the strip forming the balance spring when it is subjected to a magnetic field.
To overcome this problem of magnetic insulation of the regulating member, devices have already been proposed which protect a timepiece against the interfering influence of external magnetic fields of any type, such as for example external fields from permanent magnets or electric motors of any type.
The simplest and most radical solution also consists in completely shielding the timepiece movement to prevent penetration by any interfering field lines. This is the case proposed by CH Patent No 122391 where the watch movement is protected by a set of elements formed of a corrosion resistant alloy, with high permeability and low hysteresis, forming a magnetic screen. The elements are a depressed portion arranged between the movement and the back cover of the watch, a dust proof ring forming a dome disposed between the movement and the casing ring, and an intermediate plate disposed between the bottom plate and the dial of the watch. This method is extremely cumbersome and expensive. Indeed, it requires three extra parts which not only make the watch heavier but also increase its volume.
FR Patent No 1 408 872 discloses a lighter and less cumbersome solution than that proposed above. Here however, the watch movement is not completely surrounded with a highly permeable material, but only the back cover and periphery thereof. The device is thus formed by a case element having sufficient permeability to magnetic fields. The case element is supplemented by a soft steel casing ring with which it forms a depressed portion surrounding the movement and forming a magnetic screen. The case element is the back cover of the case, made of stainless, polishable steel with a homogeneous ferritic structure. Thus, in this embodiment, there is no addition of extra parts, since the back cover and the casing ring are actually made of highly magnetically permeable materials. Moreover, there is no screen disposed between the movement and watch dial, since the protective device is limited to a depressed portion with no cover which serves as a housing for the watch movement.
A first drawback of this latter solution however, is that the actual balance spring is not protected against an interfering field whatever the orientation of the field prevailing in the balance spring plane. Indeed, since the balance spring is off-centre relative to the centre of the movement, and if omnidirectional protection is required, a device is proposed that is centred relative to said balance spring and not relative to the movement as a whole as is the case of the aforecited document. Another drawback of this solution is that the movement is totally concealed, which is detrimental to the aesthetics of a watch, particularly for high-end watches.
Balances are also known which are made of ferromagnetic materials in electronic watches, such as, for example, in the watches described in FR Patent No 2063101 or CH Patent No 361247. The ferromagnetic material employed for the balance does not, however, constitute a magnetic shielding for improving the isochronism of the balance spring, but is intended to cooperate with an electromagnetic circuit maintaining oscillations. FR Patent No 2000706 is an example of a similar solution for an electronic watch comprising a ferromagnetic balance-regulator which has no balance spring at all.
Finally, there are known from CH Patent No 689106 balance springs made with particular alloys having advantageous elastic and thermo-elastic properties for a fastening with a nickel balance. However, no particular magnetic shielding properties are mentioned for the balance in relation to the balance spring.
Consequently, it is an object of the present invention to provide a solution which aims to improve the magnetic shielding of a balance spring and which does not have the above limitations.